1. Field of the Invention
The present invention relates to a vibrating beam accelerometer and, more particularly, to a vibrating beam accelerometer which includes an electromagnetically excited double-ended tuning fork (DETF) formed with separate conducting paths for the drive circuit and pick-off circuit to minimize the problems associated with vibrating beam accelerometers formed with a single conducting path resulting from variations of the resistance path due to manufacturing tolerances and temperature changes.
2. Description of the Prior Art
Vibrating beam accelerometers are generally known in the art. An example of a known vibrating beam accelerometer is shown in FIG. 1. Such known vibrating beam accelerometers normally include a pendulum or proof mass 20, connected to a casing 22 by way of a pair of flexures 24 to enable the pendulum 20 to rotate about a hinge axis HA, defined by the flexures 24. A double-ended tuning fork (DETF) 26, which includes two vibrating beams or tines 28 and 30, is connected between the casing 22 and the pendulum 20, perpendicular to the hinge axis HA to define a sensitive axis SA.
Excitation is applied to the DETF 26 to cause the vibrating beams 28, 30 to vibrate at a resonant frequency when the pendulum 20 is at rest. Forces applied along the sensitive axis SA cause the vibrating beams 28, 30 to go into either tension or compression which changes the resonant frequency of vibration of the beams 28, 30. This change in frequency, in turn, is used to measure the force applied along the sensitive axis SA.
Various types of excitation are known to force the vibrating beams 28, 30 to vibrate, such as electromagnetic, electrostatic, and thermal excitation. The type of excitation depends on the particular materials used for construction. For example, crystalline quartz DETFs are excited according to the piezoelectric property of the quartz. DETFs made from silicon are normally micromachined and are excited by other means, such as electrostatically or electromagnetically.
FIG. 1 illustrates a silicon micromachined vibrating beam accelerometer that includes a double-ended tuning fork that is adapted to be excited electromagnetically. In such an embodiment, the vibrating beams 28, 30 are electrically connected at one end 32 while electrodes or wire bond pads 33 and 34 are formed at the opposing ends 36 defining a conducting path. In such an embodiment, a conductive material, such as gold, is applied to the vibrating beams 28, 30 as well as the wire bond pads 33, 34 to enable electric current to flow between the wire bond pads 33 and 34 through the vibrating beams 28, 30 as shown. An externally generated magnetic field B is applied in a direction generally perpendicular to the plane of the DETF 26. The magnetic field B reacts with the currents in the vibrating beams 28 and 30 to cause a force on the beams 28 and 30. If the electrical current applied to the beams 28, 30 is alternating current (AC), the beams 28, 30 will vibrate. The vibration of the beams 28, 30 causes a voltage to be generated across the wire bond pads 33, 34 as a result of the magnetic field. This voltage, known as the pickoff voltage, in turn, is conditioned and fed back to the source of the excitation current to form an oscillator. A more detailed description of magnetic excitation is disclosed in U.S. Pat. No. 4,912,990, incorporated herein by reference.
A drive voltage is also available at the same wire bond pads 33 and 34. This drive voltage results from the excitation current flowing across the resistance of the DETF; a relatively large voltage drop relative to the pickoff voltage. In order to form an oscillator circuit, the drive voltage needs to be separated from the pickoff voltage, available across the same wire bond pads. It is known to use a bridge circuit to separate the voltages. For example, the voltage at the wire bond pads is known to be amplified to a practical level for regulation to provide an output signal and limited by an amplitude limiter, the output of which is connected to a resistor in series with the DETF. The DETF, in turn, is known to be connected between that resistor and ground to form a voltage divider. In order to separate the pickoff voltage from the drive voltage, the voltage from the voltage limiter is known to be supplied to another voltage divider that is proportioned so that the output of the two dividers is approximately the same except for the pickoff voltage. The two dividers are used to form a bridge, the output of which is the pickoff voltage.
The problem with such a configuration is that a single conducting path is used for both the drive voltage and the pickoff voltage. Thus, any variation of the resistance in such a single current path, as well as the components discussed above connected to the beams resulting from manufacturing tolerances and changes in temperature, will result in an oscillator that works very poorly, if at all.